1. Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically to a liquid crystal display device comprising a pair of substrates opposed to each other, a plurality of thin film transistors formed on one of the substrates, and a plurality of pixel electrodes driven according to the action of the thin film transistors.
2. Description of the Related Art
Liquid crystal display devices are widely used in TVs, and graphic displays. One of these liquid crystal display devices is an active matrix type liquid crystal display device which has high response speed, and many pixel electrodes. Also, active researches are in progress toward active matrix type liquid crystal display devices with higher picture quality, larger size, and color display.
In the active matrix type liquid crystal display devices, gate bus lines and data bus lines are arranged on a lower substrate. At the intersections of the gate bus lines and data bus lines, pixel electrodes and switching devices such as diodes or transistors are arranged.
Since a drive of the pixel electrodes are controlled independently by the switching devices, their high speed driving is possible, and equipping a large number of pixels and realizing large size display therein are also possible.
In a conventional active matrix type liquid crystal display device as shown in FIG. 1, a plurality of gate bus lines 2 are arranged in a first direction P, parallel to each other on a lower substrate 1. Each of gate electrodes 2A is a part of respective gate bus lines 2, wherein the gate lines 2 each is protruded perpendicularly from a predetermined location of the respective gate bus lines 2. An amorphous silicon layer 4 is arranged on top of surface the gate electrodes 2A. A plurality of data bus lines 7 are arranged in a second direction Q is perpendicular to the gate bus lines 2, parallel to each other, so as to form a matrix shape of unit cells along with the gate bus lines 2. The gate bus lines 2 and the data bus lines 7 have a gate insulation layer (not shown) interposed between them so as to be electrically insulated from each other. Each of source electrodes 7A is a part of their respective data bus lines 7, and is protruded by a predetermined distance to be overlapping with one side of the amorphous silicon layer 4. Each of drain electrode 7B overlaps with the other side of the amorphous silicon layer 4. A plurality of thin film transistors TFT comprise the gate electrodes 2A, the amorphous silicon layer 4, the source electrodes 7A and the drain electrodes 7B. A plurality of pixel electrodes 8 are arranged inside their respective unit cells, and contact with the drain electrodes 7B. Storage electrodes 2B are disposed beneath the pixel electrodes 8, forming storage capacitors therewith. The storage electrodes 2B are made of non-transparent material.
As shown in FIG. 2, an upper substrate opposite to the lower substrate 1 includes a plurality of color filters 11 having the same shape as the pixel electrodes 8 on the lower substrate 1 so as to correspond with the same. A black matrix 12 is arranged to surround the respective outer portions of the color filters 11 to prevent scattering of colors of the color filters 11, and to prevent leakage of light from the lower substrate 1. The black matrix 12 is made of a non-transparent material. A transparent electrode (not shown) is coated over the color filters 11 and the black matrix 12, and is electrically grounded along with the storage electrodes 2B.
In the liquid crystal display having a structure as described above, if a scanning selecting signal is applied to one of the gate bus lines 2, and an image signal is transmitted to a corresponding data bus line 7, thin film transistors TFT located at the intersection of the data bus line 7 and the gate bus line 2 to which the scanning selecting signals were applied are turned on, and the corresponding pixel electrode 8 is driven. Then the image signal loaded into the data bus lines 7 flows from the source electrodes 7A to the drain electrodes 7B and is then applied to the pixel electrodes 8 being in contact with drain electrodes 7B, thereby driving the pixel electrodes 8.
In a result, a predetermined electric field is formed between the pixel electrodes 8 and the grounded transparent electrodes (not shown), and the liquid crystal molecules are activated. However, at intersections(shown as X in FIG. 1) of the gate bus lines 2 and a data bus lines 7, the foregoing liquid crystal display devices have a gate insulation film(not shown) disposed only between the gate bus lines 2 and the data bus lines 7. Hence, if a slight damage should arise at these gate insulation film, short circuit between gate bus lines 2 and data bus lines 7 is apt to happen.
Also, as the intersections of gate bus lines 2 and data bus lines 7, and data bus lines 7 and storage electrodes 2B increase, parasitic capacitance also increases. This causes the signal delay time T=RC to increase. Moreover, since the storage electrodes 2B are formed on the surfaces of the pixel electrodes 7, the transmitting area of light is blocked. Hence aperture ratio of the liquid crystal displays is reduced.